A harness tensioner for a child safety seat has a lower lever part pivotable about an axis and an upper lever part above the lower lever part and pivotable about a different axis. A lower portion of the upper lever part contacts with an upper portion of the lower lever part. Harness straps of a child safety seat are coupled to the upper lever part. Upon an impact event, the lower part is moved against a biasing force, which in turn moves the upper part to apply rearward tension to the harness straps.